Linearity or UnlineariW of the F.I,D.

Linearitiit oder Unlinearit~it des F I D

Lin6arit~ ou non-lin6arit6 du D.I.F.

H. Oster / F. Opperrnann Siemens Aktiengesellschaft Wernerwerk f~ir Mel~technik, D-75 Karlsruhe

.~uramary: Quantitative analysis by means of a flame Ionization detector require exact data on the detection properties.

By measuring the electrical field and also the quantitative dependence upon the burner air and Hydrogen supply, the parameters for stable operation of FID have been Checked. Using a new gas mixing system, it has proved Possible to measure the linearity of the FID. For Methane, q'values of 7.23 �9 10 -a A. see + 0.4% with a linearity gr - ratio of 1 : 2 �9 1 0 7 have been found. For Butane, q = 9.2.10-a _-5" see +_ 0.45 % with a linearity range of 1 :4 .107 . gr

Zu~arnmenfassung: Quantitative Analysen mit einem l~lammenionisationsdetektor erfordern eine genaue Ketmt- his der Detektoreigenschaften.

burch Vermessung des elektrischen Feldes wie auch der I~tennluft. und Wasserstoffmengenabh~ingigkeit wurden die Bedingungen fiir eine einwandfreie FID Funktion ge- t~riift. Mittels einer neuen Gasmischmethode gelang es, die Linearit~it des FID zu messen. Es wurden f'tir Methan q'Werte yon 7,23,10 -a A' sac + 0,4% bei einem Line- aritatsverl~ltnis yon 1 : 2 "~r07 gefunden. Fiir Butan q~- 9,2.10-3 ~-" ~ + 0,45 % mit einem Linearitiits- Verhaltnis von 1 gr - : 4 - l0 T .

8on~naire: L'analyse quantitative au moyen d'un d6tec- teur h ionisation de flamme demande des donn6es exactes SUr les propri6t6s de d6tection.

laes param~tres pour un fonctionnement stable du D.I.F. oat 6t6 6tudi~s en mesurant le champ 61ectrique, ainsi que la d' dependance quantitative avec les d6bits d'air et

hydrog6ne fournis au brfileur, En utilisant un nouveau SYSt6rne de m61ange des gaz, il s'est r6v616 possible de r~esurer la lin6arit6 du D.I.F. Pour le m6thane, des valeurs

q de 7,23 �9 10-3 _A �9 sec -+ 0,4 % ont 6t~ trouv6es, avec g

un rapport de lin6arit6 de 1 : 2 �9 107 . Pour le butane,

q ~-9,2 �9 10-3 A �9 sec +- 0,45 %, avec un rapport de g

lin6arit6 de 1 : 4 �9 107 .

In recent times, the proportionality of flame ionisation detectors has again become a matter of some interest. This interest has been excited by the reflections upon the evaluation of chromatographic data by means of digital computers. In addition, the trend towards precise analysis has raised the question of the properties of detec- tors used in gas chromatography. As much as ever, the FID is the most suitable detector for many fields of interest. The high detection sensitivity, the quantitative proport. ional output, the small effective detector volume, are all part of its undeniable useful properties. Apart from these, the extraordinary high linearity between the sample flow and the response (in Amperes) is marked out by one section of users and called very much in question by

another [ 1-2].

It is certainly true that not all F.I.D.'s are really linear and that measurements made with different detectors can lead to different results; we are convinced of this from our own experiments. But the unlinearity may be a func- tion of other parts of the equipment or the mode of operation. What is in question is how these results are to be interpreted. A correct diagnosis of the nonlinearity will then point the way to steps for improving the total linearity of the equipment, including injection, column, flow lines and detector.

In this contribution, the properties of the FID alone shall be investigated. If we can ensure that the particular detector system can be linear to a high degree, then the search for those elements limiting the linearity of the apparatus "gas chromatograph" can be confined to the injection, column and flow line elements.

Method of Measurement In order to remove errors from the investigation of the FID system we have applied a method of measurement, which ensures that no errors creep in due to variations such as an increase or decrease in the sample to be detec- ted which could arise from adsorption or desorption in the flow lines. This 'quasistationary' method gives the required relationship between the flow of substance and the detector current directly according to the equation

P i = q ( 1 )

Chromatographia 2, 1969 Originals 251

I = Current in Amperes q = Trans fo rmat ion fac tor in A . see gr P = Quan t i ty o f the substance in gr.

t = T ime

ep = F low o f substance in gr/sec

or the equa t ion

I = q ~ (2)

The t ransformat ion fac tor q should be cons tan t for the

widest possible range o f I. Fo r quant i ta t ive analysis the

q-values related to an internal standard are o f u tmos t

interest. The absolute value o f q however is o f subordinate

interest as j udgemen t o f a de t ec to r is made no t on the

absolute value o f the signal, bu t on the signal to noise

ratio. In any case it is possible today to supply amplif iers

o f sufficiently high stabil i ty wi th high ampl i f ica t ion factors.

The results in accordance wi th equa t ion 2 are normal ly

presented in graphical form. Use o f a l inear scale is im-

practicable because o f the large range for the quant i t ies I

and ep. Most informat ive is a l in/ log presenta t ion , where

q is p lo t ted linear against log P / t .

-2 -2

Fig. I

J

�9 Equipotentials and electrical field for the FID

FID Construction

The FID used for this investigation possesses certain special constructional properties which are described below.

The burner jet consists of a Platinum tube with a inner diameter of 0.5 mm and a wall thickness of 0.25 mm. These dimensions ensure that every glow effect of the jet is also available under extreme operating conditions. In order to achieve a high electrical resistivity between jet and housing (column), the jet has been scaled into glass on the feed through side. The insulation resistance for a detector temperature of 100 ~ is 8" 10 l~ I2. The supply voltage of -300 V is fed to the jet via a precious metal spring contact.

The anode is at earth potential. It consists of a circular Platinum grid of 10 mm diameter. The grid is coarse meshed enough not to affect the gas stream in the FID significantly and thus is has no effect on the stability of the flame.

Special attention has been paid to the supply of burner air. This flows from four channels, fags onto recoil plates and is then led through several very fine meshed filters before been admitted into the burner space. This system ensures that the air flows exceptionally evenly and without turbulence from below to above through the burner space; the flame is thus undisturbed and the Oxygen needed for combustion can be taken directly from its environment and the combustion products easily removed.

The distance of the anode from the jet is fixed so that, on one side a high sensitivity, on the other side a very favourable value of signal to noise ratio is obtained. The width of the noise band is significantly smaller that 2.10-14A. The electrical field of the FID with the selected electrode distance has been measured in an electrolytic tank (Fig. 1). This indicates that, with this field distribution, all negative charge carriers coming into existance within the flame will certainly be captured by the anode.

We checked whether additional ring-shaped electrodes at anode potential would beneficially affect the field. As Figs, 2a, 2b show, this is not the case. The field in the anode region is strongly distor- ted by the ring electrodes and, especially with the arrangement show in Fig. 2a, a large part of the negative charge carriers is led past the anode. The best form for the anode is the plain electrode, without ring. Cylindrical anodes were discarded as these are always associated with a forced current and, thus, with an increase in flame noise.

�9 ~quipotent'mle und elektrisches Feld t'dr den FID

�9 Equipotentials et champ ~lectrique pour le D.I.F.

Fig. 2a, 2b

�9 Equipotentials and electrical field for the FID with additional ring-shaped electrodes at anode potential

�9 ~quipotentiale und elektrisches Feld fur den FID mit zusiitzliche~ ringf6rmigen Elektroden auf Anoden-Potential

�9 Equipotentiels et champ ~lectrique pour le D.I.F., avec des 61corrodes annulaires additionneUes au potentiel d'anode

252 Chromatographia 2, 1969 Originals

~ r ~tting the optimum conditions, the ion stream was measured a �9 of the burner air (Fig. 3) as well as of the burner gas,

H2 (Fig. 4). A medium quantity of 7.6" 10-gg/sec of Methane Was used as a sample. It turned out that the response of the FID aPProaches a saturation value with 500 ml]min air, the H2 maximum lies at 20 ml/min. The FID signal was recorded using a Siemens modulator amplifier and a potentiometrie recorder. 1%~ reading values greater than 3 �9 10-gA, a Mieroammeter from Mess~s. Keithley was used.

I fA)

6.40.

~3o.

~,2o-

6,to

Fig. 3

/ 200 ~oo ' ebo' 8bo'Idoo' ffoo" ffoo

air [ml/min]

�9 lort caxrrent as a function of the burner ak 20 ml/min H2, detector temperature 100 ~ sample gas 7.6 " 10 -9 g/seC CH 4

�9 10hens�9 als Funktion der Brennluft, 20 ml/min H2, Detek- t~ 100 ~ Prohengas 7,6 " 10 -9 g/sec CH4

�9 ~urant ionique en fonetion de l'air fourni au brtqleur; 20 roll H2, temp6rature du d6tecteur 100 ~ gaz-6chantillon

" , b �9 10-9 g/see CH4

I tAt

6

5

" ' " - ~ 0 ' ' 2 b " ' ' ' 3 ' ( 7 " Y 6 - H ., fret/rain]

�9 10n ettrtent as a function of the burner gas H2, Burner aix 600 ~H l/nlin, detector temperature 100 ~ sample gas 7.6 " 10 -9 g/see

4

�9 i~nenstrom als Funktion des Brenngases H2. Brennluft 600 ml/ n~in, betektortemperatur 100 ~ Probengas 7,6 �9 10 -9 g/see CH4

�9 ~k urant ionique en forte�9 de l'hydrog~ne fourni au brflleur; au b~leur 600 ml/min, temp&ature du d6tecteur 100 ~

gaz'echantillon 7,6 �9 10 -9 g/see CH4

Saniple Gas Product ion

For carrying out these l inearity measurements extreme Care was taken with the product ion of the sample gas mix- ture. To produce gas mixtures in pressure cylinders is very questionable. On the one hand, one must wait for a very lDng time until an equilibrium state has shown itself in

the cylinder ; on the other hand, one should not overlook how great the adsorpt ion o f the sample gas is in the wa/ls o f the cylinder. This adsorption is very noticeable for mixtures under I to 0,1%.

For our experiments, we have taker a different route ; the required gas mixture needed during the measurement is produced by a new method suggested by M. Klobe.

Figur 5 shows the system in schematic form. The kernel of the mixing apparatus is the pair o f microapertures (perforated plates), the perforat ion having a diameter o f some micrometers [ 3 - 4 ] .

,I I I I t

Fig. Sa

[ ~ FLOW- OFF-REGULATOR wAsrE-"2As J eF--~ ~'~ B3

I ~ ~FID

SAM~PLE - - ' B - ~ M2 H PRESSURE GAUGE R~ ~ D2 O THROTFL E

N~'~ " - - ] B MICRO APERTURE R REOUC,NG VALVE

Apparatus fox the continous production of gas mixtures using microapertures (Laval-jets). The method of operation is described in the text

Geriit zur kontinuierlichen Herstellung yon Gasmisehungen mit Hilfe yon Mikro-Aperturen (Laval-Jets). Die Arbeitsmethode is�9 im Text besehrieben

Appareil utilisant des miero-ouvertures (jet Laval) pour la pro- duction continue de m6langes gazeux. Le mode op6ratoixe est d6crite darts le texte

._.m,,. ~ , I

Fig. 5b SAMPLE

�9 Schematic diagram of the flow-off regulator

�9 Schematische DarsteUung des AbstriSm-Reglers

�9 Sch6matique du r6gulateur d'6coulement N2

Chromatoglaphia 2, 1969 Originals 253

5 H2 ~

-Q

Fig. 6

�9 Two-stage system for the continous production of gas mixtures in the ppm range

Q

[ml/min]

t,5-

~,0-

0,5-

�9 Zweistufen-System zur kontinuierlichen Herstellung yon Gas- mischungen im ppm-Bereich

�9 Syst~me ~ deux 6tages pour ia production continue de m61anges gazeux dans le domaine des ppm

The most noticeable property of the apertures being that they guarantee a constant flow of gas, provided that the absolute pressure in front o f the aperture is at least double that of the pressure behind the aperture. Then the flow for a certain gas is a function of the pressure in front o f the aperture and of the dimension of the aperture. This relation is shown in Fig. 7. A further property o f the aperture, significant for gas mixing, is that the flow is turbulent for a short distance beyond the aperture. If a second gas stream is introduced into this zone, then complete mixing happens very rapidly. The operation of the mixing apparatus can be clarified by means of an example : The apertures for the sample gas and for Nitrogen may be chosen so that, with corresponding pressures, the flows have a ratio o f 1 : 103ml/min. From the gas mixture thus produced, only a small quantity is required, the surplus part needing to be split off. This is achieved by a regulator which seeks to hold the pressure on its input side by blowing-off (flow-off regulator) in conjunction with a further micro-aperture. The flow-off regulator, together with the aperture in the flow line, holds a fixed, adjustable pressure which is independent of the quality of the inflowing gas mixture over a large range. By a suitable choice of aperture and the placing of two such units in series, Fig. 6, it is possible to obtain a mixture in the ppm range easily. Given precise quant i- tative knowledge of the flow, then the mixing apparatus works very reliably, the values being reproduceable as often as needed.

A further property of this mixing unit worth remarking on is the simple possibility for correcting for the impurity of the dilution gas (Nitrogen). Because of the constant pressure in front of the last throttle before the detector, the detector will be supplied with an effectively constant mixture. With very high sample gas quantities, there will be little diluting gas, with trace amounts, a lot of diluent will be fed to the detector. With an ion current of 10-gA the share due to impurity is only 0.1%, if we take an

t

n2

o ~ #o t f diameter

Fig. 7

Gas flow in ml min -i at -i through the microapertures as function of the diameter. The curves axe calculated for H2 and N2 at tool0 temperature

Gasstr6mung in ml min -1 at -I dutch die Mikro-Btenden als Funktion des Durchmessers. Die Kurven wurden fiir H2 und N~ bei Raumtemperatur berechnet

Couzant gaseux darts les micro-ouvertures en ml rain -1 a t-1 err f otlr de leur diam~tre. Les courbes sont calcul6es, ~ la temp6ratttres ambiante, pour H:z et N2

impurity of the diluting gas corresponding to 1 �9 10"l~A ion current. With greater currents (higher sample quan" tities), the current share due to impurity can, therefore, be neglected. From 10 -9 downwards to an infinite diluti0~ of the sample gas, the current value for impurity can be taken as constant. This value shows a change of only O. 1% and therefore lies within the measuring accuracY" From the consideration of this error, one sees that, for correcting the data, only the constant value of our exa~P" le (1 �9 10 -12 A) has to be subtracted from the measured

ion current. The test system was built with special regard to shortest flow lines in order to prevent possible errors from a ds~ ion and desorption in the wails of the tubing. Especially for Butane measurements, additional heating must be provided for the flow lines in order to reduce condensat" ion and adsorption. This means that our system is restriC" ted for gases as testing materials only.

R e s u l t s

Figure 8 shows the results for Methane (purity greater than 99 %), using the 'q-presentation' An average value

�9 Olo - 3 A ' s e c . . . fO4~A, of 7.23" 10 gr , with a standard deviation o �9 ..6 is obtained for The maximum 1 14 1U �9 ' q. ' ion current is �9 " a value which is fixed by the geometry of the F1D. The flame is then larger than the distance between the eleC" trodes.

254 Chromatographia 2, 1969 Originals

* = ~176 * ~ **g * ~ =o= * " %* * *ao

o *~

%. . .= .o . . , * " * B u t o n

. . ~ M e t h o n

�9 =

=%

*o

I I I ,0-9 Ib" lb-~ 1o-5 %. 8

I

7o-~ ~ [~]

q'presentation of the results for Methane (x) and Butane (0). Nitrogen: 10 ml/min; Hydrogen: 20 ml/min; burner air: 600 ml/min tletector temperature i00 ~

�9 l)arstellung Yon q aus den Ergebnissen for Methan (x) und Butan (0). Stickstoff: 10 ml/min; Wasserstoff: 20 ml/min; Brennluft: 600 rnl/min; Detektortemperatur: 100 ~

�9 Valeurs de q tir~es des r6sultats pour le rn6thane (x) et le butane (0). Azote: 10 ml/min; hydrog~ne: 20 ml/min; air au brfileur: 600 ml/min; temp&ature du d6tecteur: 100 ~

The linearity was measured over five orders of magnitude and the curve will be linearly towards the lower concent- rations as there is no reason to suppose that variations in the linearity would occur with such high dilution. At these low concentrations, i.e. partial pressures of 10-3 Torr, the molecules will certainly be within the range of ideal dilution. Here, mutual interactions between sample mole- bUrles may be nearly excluded and thus each molecule

teaks down and bums on its own. Further dilution, therefore, only reduces the number of ions formed while the kind of formation remains unaltered. We may, there- fore, assume that the linearity continues down to the lOWest detectable quantity of sample which, for this ~ etector, is 10-~4A ~ 1.4. lO-12gr/sec. This limit occurs

ecause of noise in the detector. Measurements made with Butane as sample gas are more ~ r~ than those made with Methane, as Butane

as a boiling point of 0.5 ~ Because of the low vapour Pressure at 20 ~ (ca. 1.5 atm), it was neccessary to heat that part of the mixing apparatus in which pure Butane .tlOWed. The Butane container itself was warmed to 50 ~ tn a temperature controlled water bath (control bet ter than • 0.5 ~ the tubing and apertures to 150 ~ Because of the high regulating accuracy it was possible to drop the reducing valve in the heated lines.

Figure 8 also shows the results for Butane (Butane from Va. Fluka, Buchs, 99 Mol % n-Butane) using the q-presen- tation. An average value for q of 9.2- 10 -3 A'see with g a Standard deviation o f 0.45 % was obtained. The largest value obtained for Methane in the linear range is 2" 10 -7 A; hre lowest detectable value was assumed to be 1 �9 10 -~4 A.

ona this, a linearity ratio of 1 : 2 �9 107 may be obtained.

For Butane, with a maximum value within the linearity range of 4 �9 10 -7 A, the ratio is 1 : 4 �9 107. The max imum ion current is again 1.14 �9 10 -6 A.

As a consequence of the measurements obtained, we can now say with certainty that the linearity of the apparatus "gas chromatograph" is in no way limited by the FID, at least not in the recent stated degree, provided that the FID is properly constructed. This not only implies the correct geometry for the electrode layout but also extreme care has to be taken for good flow relations. The measure- ments also show that it is not permissible to test the linearity of an FID dynamically, i.e., to test with the use

of a chromatograph. The search for those elements which limit the linearity can, therefore, be confined to the injection, the column

and the flow lines.

Literature

[1] Deans, D.R., Chromatographia 1,187 (1968). [2] Kaiser, R., Stoll, W., Fischer, K., Chromatographia 2,

20 (1969). [3] Oster, H., Chromatographia 1,144 (1968). [4] Klobe, M., ATM Blatt V, 723; 29 th May (1968).

Received: January 1, 1969 Accepted: March 12, 1969

Chromatographia 2, 1969 Originals 255